High-Hardness Martensitic Stainless Steel with Excellent Antibacterial Property and Preparation Method Therefor

ABSTRACT

A high-hardness martensitic stainless steel with excellent antibacterial property and a preparation method therefor are disclosed. The high-hardness martensitic stainless steel with excellent antibacterial property comprises: 0.45-0.65 wt % of C; 0.02-0.06 wt % of N; 0.1-0.6 wt % of Si; 0.3-1.0 wt % of Mn; 0.1-0.4 wt % of Ni; 13-14.5 wt % of Cr; 0.4-0.6 wt % of Mo; 0.8-1.2 wt % of W; 1.5-2.0 wt % of Cu; and the balance of Fe and inevitable impurities. According to the present disclosure, there is an advantage enabling the preparation of the martensitic stainless steel for knives, the martensitic stainless steel having high hardness, high corrosion resistance and excellent antibacterial property, by uniformly distributing fine chromium carbide and e-Cu precipitates in the microstructure of a batch annealed material of a high-carbon martensitic stainless steel containing Cu. In addition, there is an advantage of causing no rust formation on a material after an antibacterial evaluation.

TECHNICAL FIELD

The present disclosure relates to a high-hardness martensitic stainless steel having excellent antibacterial properties and a method for producing thereof.

BACKGROUND ART

With the recent improvement of standards of living, users' interest in hygiene and safety is increasing. Thus, there is a need to develop antibacterial stainless steels with enhanced hygienic properties that may inhibit the growth of bacteria such as Escherichia coli and Staphylococcus aureus as well as inhibit rust, which is the most important characteristic of stainless steels.

As the methods of imparting antibacterial properties to stainless steels, a method of adding a metallic element such as Ag or Cu to stainless steels has been widely used to express antibiosis.

Although Ag is known to exhibit better antibacterial properties than to Cu, Ag is expensive, causes deterioration of corrosion resistance, and is difficult to be uniformly dispersed/distributed in the stainless structure due to characteristics of elements having a lower solidified amount and a larger specific gravity.

It has been reported that Cu has an excellent antibacterial property when it is added to a stainless steel at a certain amount or more because of its low cost compared to Ag and excellent antibacterial property.

The mechanism of antibacterial action of a Cu-containing stainless steel is summarized as follows.

In the case of stainless steel with a certain amount of Cu added, the Cu element present in the surface layer is ionized by moisture on the surface of the steel in a small amount to activate Cu²⁺ ions. The activated Cu²⁺ ions may slow down the activity of SH-group-containing enzymes required for normal reactions of bacteria such as Escherichia coli and Staphylococcus aureus, thereby ultimately killing the bacteria and enhancing hygienic properties.

On the other hand, in the case of STS steels, since a passivated layer having a high density is formed on the surface, the amount of Cu²⁺ ions that may be eluted in the form of ions through contact with water by the solidified Cu atoms is extremely limited.

Thus, not only the antibacterial properties are degraded but also the duration of antibacterial activity is shortened.

In order to solve this problem, in recent years, a method of precipitating fine Cu-rich precipitates (e-Cu) by subjecting manufactured steel sheets to predetermined heat treatment for a given time at a proper temperature range has been proposed.

In this case, because Cu²⁺ ion elution is activated from the e-Cu precipitation phase partially protruding from the surface layer by the predetermined heat treatment, improved antibacterial properties may be stably maintained for a long time.

Shaking flask methods and film adhesion methods are most widely used as methods for evaluating antibacterial properties.

The shaking flask method is mainly used for waterproof/water repellent materials, materials with high surface roughness, and materials with high absorbency and the film adhesion method is mainly used for materials that are smooth and not absorbent.

In the case of metallic materials, the antibacterial properties are mainly evaluated by using the film adhesion method generally by applying JIS Z 2801 standard. If the antibacterial properties are evaluated according to the JIS Z 2801 standard, the bacteria are cultured for 24 hours using an inoculum containing 0.5-0.85% NaCl. When testing under these conditions, corrosion such as rust occurs in the material with low corrosion resistance.

The reliability of the evaluation result of the antibacterial properties of the materials may deteriorate when corrosion occurs.

Therefore, it is essential to secure corrosion resistance so that no corrosion phenomenon is observed after the cultivation of bacteria in the evaluation of the antibacterial properties.

DISCLOSURE Technical Problem

Provided are a high-hardness martensitic stainless steel having excellent antibacterial properties in which a chromium carbide is uniformly distributed in a microstructure thereof without having rust or corrosion after evaluation of antibacterial properties, and a method of producing the same.

Additional aspects of the disclosure will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the disclosure.

Technical Solution

According to an aspect of an embodiment, a high hardness martensitic stainless steel having excellent antibacterial properties includes 0.45 to 0.65 wt % of carbon (C); 0.02 to 0.06 wt % of nitrogen (N); 0.1 to 0.6 wt % of silicon (Si); 0.3 to 1.0 wt % of manganese (Mn); 0.1 to 0.4 wt % of nickel (Ni); 13 to 14.5 wt % of chromium (Cr); 0.4 to 0.6 wt % of molybdenum (Mo); 0.8 to 1.2 wt % of tungsten (W); 1.5 to 2.0 wt % of copper (Cu); and the balance of iron (Fe) and inevitable impurities.

The martensitic stainless steel may be subjected to a batch annealing treatment to have an elongation of 18% or more.

The batch annealing process may be performed to distribute 90 or more chromium carbides (100 pmt) in a structure of the martensitic stainless steel.

The martensitic stainless steel may satisfy the following Pitting Resistance Equivalent Number (PREN) and does not cause surface deterioration during evaluation of antibacterial activities using an inoculum containing NaCl, and may exhibit a bacterial reduction rate of 99% or more.

Pitting Resistance Equivalent Number (PREN) Cr+3.0(Mo+½W)+16N≧17

The batch annealing process may include a first soaking process to uniformly distribute Cu precipitates in a structure of the martensitic stainless steel, a second soaking process to uniformly distribute a chromium carbide in the structure of the martensitic stainless steel; and a third soaking process to spheroidize fine particles of the chromium carbide.

The first soaking process may be performed at 500 to 600° C., the second soaking process may be performed at 800 to 900° C., and the third soaking process may be performed at 600 to 750° C.

The first soaking process may continue for 5 to 15 hours, the second soaking process may continue for 15 to 25 hours, and the third soaking process may continue for 5 to 15 hours.

The batch annealing process may further include a heating process of elevating temperature at a rate of 40 to 200° C./h until the second soaking process after the first soaking process, a cooling process of lowering temperature at a rate of 10° C./h or higher until the third soaking process after the second soaking process, and an air-cooling process performed after the third soaking process.

According to an aspect of another embodiment, a method of producing a high-hardness martensitic stainless steel having excellent antibacterial properties includes producing a hot-rolled steel sheet by hot rolling a cast strip, softening the hot-rolled steel sheet by a batch annealing process, and producing a cold-rolled steel sheet by cold rolling the annealed steel sheet treated by the batch annealing process, wherein the batch annealing process comprises: a first soaking process to uniformly distribute Cu precipitates in a structure of the martensitic stainless steel; a second soaking process to uniformly distribute a chromium carbide in the structure of the martensitic stainless steel; and a third soaking process to spheroidize fine particles of the chromium carbide.

Advantageous Effects

According to the present disclosure, a martensitic stainless steel material, which has high hardness, high corrosion resistance, and excellent antibacterial properties, may be manufactured by uniformly distributing fine chromium carbide and e-Cu precipitation phases in a microstructure of an annealed high carbon martensitic stainless steel including Cu.

Further, according to the present disclosure corrosion phenomenon does not occur in a material after the evaluation of antibacterial property.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a batch annealing process according to an embodiment of the present disclosure;

FIG. 2 illustrates photographs showing microstructures of Cu precipitates included in structures according to the content of Cu in the first soaking process according to an embodiment of the present disclosure; and

FIG. 3 illustrates photographs showing surfaces of materials after antibacterial property tests according to an embodiment of the present disclosure.

BEST MODE

According to a preferred embodiment of the present disclosure, provided is a high-hardness martensitic stainless steel with excellent antibacterial properties including: 0.45 to 0.65 wt % of carbon (C); 0.02 to 0.06 wt % of nitrogen (N); 0.1 to 0.6 wt % of silicon (Si); 0.3 to 1.0 wt % of manganese (Mn); 0.1 to 0.4 wt % of nickel (Ni); 13 to 14.5 wt % of chromium (Cr); 0.4 to 0.6 wt % of molybdenum (Mo); 0.8 to 1.2 wt % of tungsten (W); 1.5 to 2.0 wt % of copper (Cu); and the balance of iron (Fe) and inevitable impurities.

MODE FOR INVENTION

The present disclosure may be variously modified and includes various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. However, it is to be understood that the present disclosure is not intended to be limited to the specific embodiments but includes all changes, equivalents, and alternatives falling within the spirit and scope of the present disclosure. In describing each of the figures was used for the similar reference numerals to like elements.

Hereinafter, embodiments according to the present disclosure will be described in detail with reference to the accompanying drawings.

In general, edge tools widely used in our daily lives such as knives, scissors, razors, and scalpels which are medical instruments require high hardness in order to maintain high cutting performance and abrasion resistance and excellent corrosion resistance because they are used in contact with moisture or stored in a humid atmosphere.

Accordingly, martensitic stainless steel to which high carbon has been added is mainly used as edge tools.

As edge tools, high carbon martensitic steels for edge tools, steels including 0.45 to 0.70% by weight of carbon (C), 1.0% by weight or more of manganese (Mn), 1.0% by weight or more of silicon (Si), and 12.0 to 15.0% by weight of chromium (Cr) are have been widely used.

A method of manufacturing high-carbon martensitic stainless steels for edge tools includes a batch annealing process.

During the batch annealing process, fine particles of chromium carbide are precipitated and distributed in a ferrite matrix as a result of reactions between carbon and chromium. As a solid carbon content in the matrix decreases, the resultant material may be easily applied to a stainless steel manufacturing process such as rolling and acid pickling.

Besides, the fine chromium carbide particles uniformly distributed in the ferrite matrix microstructure enable rapid resolidification of chromium and carbon to an austenite phase of high temperature during the hardening heat treatment process performed by an edge tool manufacturer and improve hardness and corrosion resistance of the microstructure of the martensitic stainless steel after quenching.

In order to obtain a high-carbon martensitic steel for edge tools having excellent hardness and corrosion resistance, it is essential to uniformly distribute the fine chromium carbide particles in the microstructure.

Meanwhile, as described above, metallic materials may have rust or corrosion in the evaluation of antibacterial properties. Even in the case of the high-carbon martensitic steel for edge tools, the reliability of the results of the evaluation of antibacterial properties may decrease.

As patent documents related to antibacterial martensitic stainless steels, Japanese Patent Publication Nos. H9-195016 and H9-256116 disclose a martensitic stainless steel having excellent antibacterial properties in which the e-Cu precipitate phase is uniformly distributed, and a production method thereof. However, it was confirmed that there was no information on the factors that would have a great influence on the antibacterial properties such as rusting of the material, and the bacterial reduction rate when the antibacterial property was evaluated.

Therefore, in order to develop a high-hardness martensitic stainless steel having excellent antibacterial properties, it is essential to distribute the chromium carbide uniformly in the microstructure thereof and to secure corrosion resistance to prevent corrosion in the material after antibacterial evaluation.

The present disclosure relates to a high hardness martensitic stainless steel having excellent antibacterial properties including 0.45 to 0.65 wt % of carbon (C); 0.02 to 0.06 wt % of nitrogen (N); 0.1 to 0.6 wt % of silicon (Si); 0.3 to 1.0 wt % of manganese (Mn); 0.1 to 0.4 wt % of nickel (Ni); 13 to 14.5 wt % of chromium (Cr); 0.4 to 0.6 wt % of molybdenum (Mo); 0.8 to 1.2 wt % of tungsten (W); 1.5 to 2.0 wt % of copper (Cu); and the balance of iron (Fe) and inevitable impurities and having a bacterial reduction rate of 99.9% or more measured according to the Japanese Industrial Standard (JIS) Z 2801 antibacterial evaluation method.

The content of the alloy element constituting the high carbon martensitic stainless steel for edge tools according to the present disclosure will be described.

As the content of C decreases, hardness of the martensitic stainless steel is lowered after the hardening heat treatment, so that machinability and abrasion resistance may not be ensured. Thus, 0.45% or more of C is added thereto.

On the other hand, if the content of C increases, excessive formation of chromium carbide may deteriorate corrosion resistance of the material and a risk of formation of coarse carbides increases in the annealed structure due to carbon segregation. Thus, the content of C is limited up to 0.65%

As N is an element added to improve corrosion resistance and hardness at the same time, local fine segregation does not occur and thus not forming coarse precipitates in a product when added thereto in place of C. However, an excess of N may cause generation of pores during casting, and the content of N is limited up to 0.06%.

Because Si is an element essentially added for deoxidation, 0.1% or more of Si is added. However, since an excess of Si lowers acid picking performance and increases brittleness of the material, the content of Si is limited up to 0.6%.

Mn is also an element essentially added for deoxidation, and thus 0.3% or more of Mn is added. However, an excess of Mn may deteriorate the surface quality of the steel and decrease hardness of a final heat treated material since austenite remains therein. Thus, the content of Mn is limited up to 1.0%.

In the steel making process, 0.1% or more of Ni, which is inevitably brought in from scrap iron, is added. However, a high content of Ni may cause austenite to remain in the final heat treated material and it is difficult to secure high hardness. Therefore, the content of Ni is limited up to 0.4%.

Since Cr is a basic element for securing corrosion resistance, 13% or more is added. However, an excess of Cr may increase manufacturing costs and increase fine segregation of the chromium component in the structure to cause local coarsening of chromium carbide, thereby reducing corrosion resistance and hardness of the hardening heat treated material. Thus, the content of Cr is limited up to 14.5%.

Mo has an excellent effect on improving corrosion resistance, and thus 0.4% or more of Mo is added. However, an excess of Mo may increase manufacturing costs. Thus, the content of Mo is limited up to 0.6%.

W improves corrosion resistance and heat treatment hardness, 0.8% or more of W is added. However, an excess of W may increase manufacturing costs and decrease processibilty, and thus the content of W is limited up to 1.2%.

As the most important alloying element in the stainless steel of the present disclosure, Cu may ensure antibacterial properties via formation of e-Cu by the batch annealing process. Also, the higher the content, the greater the amount of e-Cu precipitates, thereby increasing an elution amount of Cu²⁺ resulting in improvement of the antibacterial properties. However, an excess of Cu may deteriorate workability, processibilty, and corrosion resistance. Thus, the content of Cu is limited up to 2.0%.

The martensitic stainless steel according to one embodiment of the present disclosure having the above composition is produced by preparing a cast strip by continuous casting or steel ingot casting, and hot rolling the cast strips to prepare a hot-rolled steel sheet which may be processed.

The hot-rolled steel sheet produced thereafter is subjected to a softening treatment through batch annealing treatment to ensure high workability before performing processing such as fine rolling to obtain a thickness suitable for edge tools.

As shown in FIG. 1, the batch annealing process according to an embodiment of the present disclosure includes a first soaking process in which spherical Cu precipitates are uniformly distributed in the structure of martensitic stainless steel, a second soaking process in which chromium carbide is uniformly distributed in the structure of the hot-rolled steel sheet, and a third soaking process to spheroidize fine particles of the chromium carbide.

Moreover, the batch annealing process may further increase a heating process of elevating temperature of the hot-rolled steel sheet until the second soaking process after the first soaking process, a cooling process of lowering the temperature of the hot-rolled steel sheet until the third soaking process after the second soaking process, and an air-cooling process of cooling the hot-rolled steel sheet after the third soaking process.

First, the first soaking process according to an embodiment of the present disclosure is a process of uniformly distributing Cu precipitates in the hot-rolled steel sheet structure performed by uniformly heating the hot-rolled steel sheet for 5 to 15 hours in a constant temperature atmosphere of 500 to 600° C.

In this process, fine Cu precipitates are uniformly distributed in the structure as shown in (b) of FIG. 2 and have a size of several tens nm. This Cu precipitates generally act as a deposition point of chromium carbide, which is well known to precipitate first in the grain boundary, and then induce precipitation of uniform chromium carbide in the second soaking process.

In the first soaking process, when the temperature is less than 500° C., no Cu precipitate is formed. When the temperature exceeds 600° C., the chromium carbide precipitates simultaneously with the Cu precipitates, whereby the chromium carbide precipitates preferentially in the grain boundaries irrespective of the Cu precipitates, so that a uniform distribution of the fine carbides may not be ensured.

Also, when the first soaking process is performed for less than 5 hours, it is impossible to secure uniform distribution of chromium carbide due to no Cu precipitation. When the first soaking process is performed for longer than 15 hours, the size of the Cu precipitates increases but the number of the Cu precipitates decreases. Thus, Cu precipitates are locally distributed, resulting in difficulty in obtaining uniform distribution of chromium carbide.

Thus, the first soaking process may be performed for 5 to 15 hours in a constant temperature atmosphere of 500 to 600° C.

Next, the heating process according to an embodiment of the present disclosure is a process of raising temperature of the hot-rolled steel sheet at a rate of 40 to 200° C./h until the second soaking process after the first soaking process.

If the heating rate is 40° C./h or less during the heating process, a temperature range during which the chromium carbide becomes coarse, for example, a time required for passing through 700 to 750° C. increases. Thus, the density of the chromium carbide distributed in the microstructure may decrease as the size of the chromium carbide increases.

On the other hand, if the heating rate is 200° C./h or more, it is fine chromium carbide may be obtained as the durability time of the temperature section where the chromium carbide is coarsened is reduced. However, a carbide diffusion time is reduced to cause nonuniform distribution of the chromium carbide.

The heating rate in the heating process may be controlled greater than 40° C./h and less than 200° C./h.

Continuously, the second soaking process according to an embodiment of the present disclosure is performed after the heating process to uniformly distribute the chromium carbide in the structure of the hot-rolled steel sheet. The second soaking process is performed by uniformly heating the hot-rolled steel sheet for 15 to 25 hours at a constant temperature atmosphere of 800 to 900° C. In this process, the chromium carbide is uniformly distributed in the structure.

If a soaking temperature is less than 800° C., agglomerates may be formed due to the chromium carbide precipitated locally at the grain boundary during the soaking process. If the soaking temperature is greater than 900° C., the chromium carbide coarsened in the vicinity of grain boundaries. Such chromium carbide agglomerates and coarse chromium carbides cause local imbalance of the material, making it difficult to secure ductility and causes material quality deterioration during a final heat treatment.

Also, if a soaking time of the second soaking process is 15 hours or less, fine particles of chromium carbide may be efficiently formed, but the chromium carbide particles may aggregate and be nonuniformly distributed. If the soaking time is longer than 25 hours, adjacent chromium carbide particles are combined due to over-annealing, chromium carbide is locally coarsened, processing efficiency decreases due to the increase of the heat treatment time, and manufacturing costs thereof increase.

Thus, the second soaking process may be performed at a constant temperature atmosphere of 800 to 900° C. for 15 to 25 hours.

Next, the cooling process according to an embodiment of the present disclosure is a process of cooling the hot-rolled steel sheet to 600 to 750° C. until the third soaking process after the second soaking process. The cooling process may be performed by cooling the hot-rolled steel sheet at a rate exceeding 10° C./h. If a cooling rate is below 10° C./h, a time required for passing through a temperature range during which the chromium carbide becomes coarse increases. Thus, fine particles of chromium carbide are coarsened in the microstructure, making it difficult to secure corrosion resistance and hardness during the hardening heat treatment.

The third soaking process according to an embodiment of the present disclosure is a process of spheroidizing the fine particles of the chromium carbide in the hot-rolled steel sheet at a low temperature followed by the cooling process and is performed by maintaining and uniformly heating hot-rolled coils in a constant temperature range of 600 to 750° C. for 5 to 15 hours.

A minimum temperature condition for spheroidizing the chromium carbide is 600° C. If the temperature exceeds 750° C., spheroidized chromium carbide grows excessively, so that the number of chromium carbide decreases and ductility decreases.

Furthermore, when the constant temperature maintaining time of the third soaking process is less than 5 hours, the chromium carbide is inadequately spherodized. When the constant temperature treatment time is longer than 15 hours, spheroidized carbide may forms a coarse microstructure by excessive growth.

Thus, the third soaking process may be performed at a constant temperature atmosphere of 600 to 750° C. for 5 to 15 hours.

After the third soaking process, the batch annealing process is completed by air cooling the hot-rolled steel sheet after the third soaking process.

After completing the softening through the above-mentioned batch annealing process, a process of preparing the cold-rolled steel sheet is performed by cold-rolling the annealed steel sheet, and a hardening heat treatment is performed on the cold-rolled steel sheet processed to have a desired thickness and shape.

The hardening heat treatment is carried out in three stages, the first stage being osteonizing→quenching heat treatment to resolidification the carbide uniformly distributed by the batch annealing process.

The first stage, is performed by heat treatment at 1000 to 1150° C. for 10 seconds to 5 minutes. Here, when a heat treatment temperature is less than 1000° C., a hardness required for a steel for shaving blades is not obtained. When the heat treatment temperature is higher than 1150° C., a large amount of austenite remains due to an increase in the amount of resolidified carbide, thereby decreasing hardness.

Also, when the heat treatment time is less than 10 seconds, hardness required by the steel material for a blade is not obtained, and even if the heat treatment time exceeds 5 minutes, grains may grow, thereby causing residual austenite.

After the quenching heat treatment is completed, a subzero heat treatment is performed at a temperature of about −70° C. for 10 seconds to 5 minutes to transform some of the remaining austenite into martensite. To ensure ductility of the martensitic steel, a tempering process is performed at 400 to 600° C. for 30 minutes to 2 hours, followed by air cooling to complete the hardening heat treatment process.

Hereinafter, one or more exemplary embodiments will be described in detail with reference to the following examples and comparative examples. However, these examples are not intended to limit the purpose and scope of the one or more exemplary embodiments.

First, hot-rolled steel sheets containing the compositions shown in Table 1 below and the balance of iron (Fe) and inevitable impurities (wt %) were prepared according to the examples and comparative examples.

As a reference, corrosion resistance of the stainless steel was quantified by using Pitting Resistance Equivalent Number (PREN, Equation 1), which is one of the methods for evaluating corrosion resistance of stainless steel products.

PREN=Cr+3.0(Mo+½W)+16N  Equation 1:

TABLE 1 Stainless steel C Si Mn Cr Ni Mo W Cu N PREN Comparative 0.683 0.408 0.693 13.21 0.308 0 0 0 0.03 13.69 example 1 Comparative 0.687 0.402 0.730 13.24 0.296 0 0 0.51 0.04 13.88 example 2 Comparative 0.651 0.4 0.688 13.3 0.299 0 0 1.02 0.06 14.26 example 3 Comparative 0.700 0.380 0.638 12.76 0.310 0 0 1.46 0.03 13.24 example 4 Comparative 0.692 0.424 0.722 13.55 0.302 0 0 1.98 0.05 14.35 example 5 Comparative 0.695 0.391 0.701 13.16 0.3 0 0 2.52 0.04 13.8 example 6 Comparative 0.49 0.305 0.517 13.98 0.307 0.51 1.05 0 0.031 17.69 example 7 Comparative 0.56 0.313 0.472 13.81 0.295 0.49 1.01 0 0.029 17.36 example 8 Comparative 0.62 0.298 0.505 14.01 0.308 0.51 1.03 0 0.03 17.67 example 9 Comparative 0.66 0.312 0.528 13.92 0.310 0.48 1.02 1.50 0.028 17.44 example 10 Example 1 0.45 0.297 0.489 13.91 0.287 0.48 0.99 1.52 0.029 17.40 Example 2 0.5 0.299 0.506 14.14 0.302 0.49 0.95 1.5 0.03 17.61 Example 3 0.56 0.298 0.509 14 0.301 0.5 1 1.52 0.03 17.58 Example 4 0.6 0.291 0.503 13.95 0.305 0.49 0.98 1.5 0.03 17.47

After cast strips of high carbon martensitic stainless steel for edge tools having the compositions shown in Table 1 were produced, hot-rolled steel sheets (thickness: 3 mm) were produced by hot rolling, and edge quality of the hot-rolled steel materials was confirmed.

Thereafter, the produced hot-rolled steel sheets were heat-treated under the following batch annealing conditions, and then observation of microstructures thereof and elongation evaluations were carried out.

[Batch Annealing Process Conditions]

-   -   First soaking process: at 500° C. for 10 hours     -   Heating process: at a heating rate of 100° C./hr     -   Second soaking process: at 850° C. for 20 hours     -   Cooling process: at a temperature lowering rate 10° C./hr     -   Third soaking process: at 650° C. for 7 hours

Next, cold-rolled steel sheets (thickness: 1.5 mm) were produced through cold rolling and the edge quality of the cold-rolled materials was confirmed.

Furthermore, after hardening heat treatment was carried out under the following conditions, antibacterial properties were evaluated using one strain (Escherichia coli) according to JIS Z 2801.

[Hardening Heat Treatment Conditions]

-   -   Austenitizing: at 1100° C. for 5 minutes     -   Quenching: oil quenching at room temperature     -   Deep freezing: at −70° C. for 5 minutes     -   Tempering/Sintering: at 500° C. for 30 minutes

In addition, surface observation was carried out to analysis the presence or absence of corrosion on the evaluated materials.

TABLE 2 Batch annealed material Hardening heat Hot Precipitate Cold treatment material rolling uniformity rolling Bacterial Stainless edge elongation Cr Cu edge Surface reduction steel quality (%) carbide precipitates quality corrosion rate (%) Comparative good 21.1 low low good ◯ 99.9 example 1 Comparative good 20.8 low low good ◯ 99.9 example 2 Comparative good 19.3 low low good ◯ 99.9 example 3 Comparative good 19.2 good good good ◯ 99.9 example 4 Comparative good 19.2 good good good ◯ 99.9 example 5 Comparative low 17.5 good good low ◯ 99.9 example 6 Comparative good 21.4 good good good X 92.5 example 7 Comparative good 21.1 good good good X 94.1 example 8 Comparative good 20.4 good good good X 94.4 example 9 Comparative good 17.3 good good low X 99.9 example 10 Example 1 good 21.6 good good good X 99.9 Example 2 good 20.1 good good good X 99.9 Example 3 good 19.8 good good good X 99.9 Example 4 good 19.4 good good good X 99.9

When the Cu content in the hot-rolled steel sheet is 0 to 2.0%, the quality of the surface and edge of the material are satisfactory after hot rolling, whereas when the Cu content is 2.5% or more (Comparative Example 6), it was confirmed that a large amount of cracks was generated. It is considered that this is caused by deterioration of hot workability due to the addition of a large amount of Cu. In addition, elongation was less than 18% even after the batch annealing process.

Based on the above results, it may be seen that the Cu content should be limited to 2% or less in order to ensure good hot workability.

On the other hand, in the case of adding Mo, W or the like (Comparative Examples 7 to 10 and Examples 1 to 4) for improving the corrosion resistance, good hot rolling property is exhibited regardless of the amount of C added of 0.45 to 0.70%. On the contrary, it was confirmed that when the C content exceeds 0.65% during the cold rolling after the batch annealing process, a large amount of cracks was generated at the edge of the cold-rolled steel sheet and the elongation rate after cold rolling was as low as less than 18%. It is considered that this is caused not only by coarse carbide formed due to an excess of C but also by precipitates of additional elements such as W and Cu.

Based on the above results, it may be seen that the content of C should be limited to 0.65% or less in order to secure good cold workability.

In addition, chromium carbide and Cu precipitation phase were observed by observing microstructure of annealing material.

First, in Comparative Examples 1 to 6, it was confirmed that the uniformity of chromium carbide was increased as the content of Cu increased from 0 to 2.5% at a constant C content. In particular, when the Cu content is more than 1.5%, it is possible to secure a density of carbide of 90/100 μm² or more in the matrix and to secure high hardness and excellent corrosion resistance after hardening heat treatment by customers.

On the other hand, it was confirmed that when the Cu content to be added was increased by 1.5% or more, the Cu precipitates distributed in the annealed structure were uniformly distributed as shown in (b) of FIG. 2.

In more detail, when the Cu content is less than 1.5% as shown in (a) of FIG. 2, formation of a nonuniform Cu precipitation phase may deteriorate antibacterial properties. On the contrary, in the case of (b) of FIG. 2 in which 1.5% or more of Cu is added, the Cu precipitation phase is uniformly distributed in the matrix, and thus it would be possible to exhibit excellent antibacterial properties.

Based on the above results, in order to secure high hardness, excellent corrosion resistance, and excellent antibacterial properties, the amount of Cu should be 1.5% or more, but the Cu content should be limited to 1.5 to 2.0% for high workability of the material.

After completion of the hardening heat treatment using the hot-rolled steel sheet, the antibacterial properties were evaluated, and then the presence or absence of rust or corrosion on the surface of the materials was confirmed.

First, evaluation of antibacterial properties with respect to the amount of Cu for the materials of Comparative Examples 1 to 6 to which Mo and W were not added and surface corrosion phenomenon of the evaluated materials was observed. As a result, it was confirmed that antibacterial properties were as high as 99.9% regardless of the amount of Cu added.

However, as a result of observing the surface layer of the materials that have been evaluated, it was confirmed that the surface corrosion phenomenon according to the corrosion resistance degradation deepened as shown in (a) of FIG. 3. Thus, it is unclear whether the expression antibacterial activities are caused by the influence of Cu contained in the material itself or the influence of Fe ions eluted from rust or corrosion.

Therefore, in order to obtain a reliable quantitative evaluation result, it is required to improve the corrosion resistance of a material which does not cause surface layer deterioration such as inhibition of the corrosion phenomenon of the material during the evaluation of antibacterial properties.

On the other hand, in the case where a steel sheet to which a certain amount of Mo or W was added (Comparative Examples 7 to 10 and Examples 1 to 4) for improving the corrosion resistance, it was confirmed that no surface layer decomposition such as a corrosion phenomenon was formed in the material after the antibacterial evaluation, as shown in (b) of FIG. 3. That is, the components of Cr, Mo, W, N and the like are set such that the value of Pitting Resistance Equivalent Number (PREN) expressed in the above Equation 1 is 17 or more, thus it is confirmed that no corrosion phenomenon is observed in the evaluation of antibacterial properties.

It is verified that in the case of a steel sheet not containing Cu (Comparative Examples 7 to 9), the rate of degraded bacterial reduction of less than 95% was shown, whereas in the case of the steel sheet to which the content of Cu is added by 1.5% or more (Comparative Example 10 and Examples 1 to 4), excellent antibacterial property of 99.9% was shown, as a result of evaluating the antibacterial properties of the material having improved corrosion resistance.

Based on the above results, it may be seen that when the PREN value is set to 17 or more by adding an element for improving the corrosion resistance of a material such as Mo, W. When 1.5% or more of Cu is added, not only excellent antibacterial properties may be secured, but also the corrosion phenomenon is suppressed after the antibacterial properties is evaluated, so that the reliable antibacterial properties may be obtained in order to secure excellent antibacterial properties.

From the above results, when the batch annealing process and hardening heat treatment under the conditions described according to the present disclosure are applied to the martensitic stainless steel including 0.45 to 0.65 wt % of carbon (C); 0.02 to 0.06 wt % of nitrogen (N); 0.1 to 0.6 wt % of silicon (Si); 0.3 to 1.0 wt % of manganese (Mn); 0.1 to 0.4 wt % of nickel (Ni); 13 to 14.5 wt % of chromium (Cr); 0.4 to 0.6 wt % of molybdenum (Mo); 0.8 to 1.2 wt % of tungsten (W); 1.5 to 2.0 wt % of copper (Cu); and the balance of iron (Fe) and inevitable impurities, a corrosion phenomenon was not observed after evaluation of antibacterial properties according to JIS Z 2801 and excellent antibacterial properties with a bacterial reduction rate of 99.9% or more were obtained.

As described above, the present disclosure has been described with reference to particular embodiments, such as specific constituent elements, and limited embodiments and drawings, but it is to be understood that the present disclosure is not limited to the above-described embodiments, and various modifications and variations may be made thereto by those skilled in the art in the field of the present disclosure. Accordingly, the spirit of the present disclosure should not be construed as being limited to the embodiments described, and all the equivalents or equivalents of the claims, as well as the following claims, fall within the scope of the present disclosure. 

1. A high-hardness martensitic stainless steel with excellent antibacterial properties comprising: 0.45 to 0.65 wt % of carbon (C); 0.02 to 0.06 wt % of nitrogen (N); 0.1 to 0.6 wt % of silicon (Si); 0.3 to 1.0 wt % of manganese (Mn); 0.1 to 0.4 wt % of nickel (Ni); 13 to 14.5 wt % of chromium (Cr); 0.4 to 0.6 wt % of molybdenum (Mo); 0.8 to 1.2 wt % of tungsten (W); 1.5 to 2.0 wt % of copper (Cu); and the balance of iron (Fe) and inevitable impurities.
 2. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 1, wherein the martensitic stainless steel has an elongation of 18% or more.
 3. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 2, wherein a chromium carbide is distributed in a martensitic stainless steel structure in an amount of 90 pieces/100 μm² or more.
 4. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 3, wherein the martensitic stainless steel satisfies Pitting Resistance Equivalent Number (PREN) described below, does not have surface deterioration during evaluation of antibacterial activities using an inoculum containing NaCl, and exhibits a bacterial reduction rate of 99% or more: Pitting Resistance Equivalent Number (PREN) Cr+3.0(Mo+½W)+16N≧17.
 5. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 2, wherein a batch annealing process to prepare the martensitic stainless steel comprises: a first soaking process to uniformly distribute Cu precipitates in a structure of the martensitic stainless steel; a second soaking process to uniformly distribute the chromium carbide in the structure of the martensitic stainless steel; and a third soaking process to spheroidize fine particles of the chromium carbide.
 6. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 5, wherein the first soaking process is performed at 500 to 600° C., the second soaking process is performed at 800 to 900° C., and the third soaking process is performed at 600 to 750° C.
 7. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 6, wherein the first soaking process continues for 5 to 15 hours, the second soaking process continues for 15 to 25 hours, and the third soaking process continues for 5 to 15 hours.
 8. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 7, wherein the batch annealing process further comprises: a heating process of elevating a temperature at a rate of 40 to 200° C./h until the second soaking process after the first soaking process; a cooling process of lowering the temperature at a rate of 10° C./h or higher until the third soaking process after the second soaking process; and an air-cooling process performed after the third soaking process.
 9. A method of producing a high-hardness martensitic stainless steel with excellent antibacterial properties, the method comprising: producing a hot-rolled steel sheet by hot rolling a cast strip; softening the hot-rolled steel sheet by a batch annealing process; and producing a cold-rolled steel sheet by cold rolling the annealed steel sheet treated by the softening process, wherein the batch annealing process comprises: a first soaking process to uniformly distribute Cu precipitates in a structure of the martensitic stainless steel; a second soaking process to uniformly distribute a chromium carbide in the structure of the martensitic stainless steel; and a third soaking process to spheroidize fine particles of the chromium carbide.
 10. The method for producing a high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 9, wherein the first soaking process is performed for 5 to 15 hours at a temperature of 500 to 600° C.; the second soaking process is performed for 15 to 25 hours at a temperature of 800 to 900° C.; and the third soaking process is performed for 5 to 15 hours at a temperature of 600 to 750° C.
 11. The method for producing a high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 10, wherein the batch annealing process further comprises: a heating process of elevating a temperature at a rate of 40 to 200° C./h until the second soaking process after the first soaking process; a cooling process of lowering the temperature at a rate of 10° C./h or higher until the third soaking process after the second soaking process; and an air-cooling process performed after the third soaking process.
 12. The method for producing a high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 11, further comprising performing a hardening heat treatment on the cold rolled steel sheet, wherein the hardening heat treatment comprises austenitizing the cold-rolled steel sheet at 1000 to 1150° C. for 10 seconds to 5 minutes to re-solidify the uniformly distributed chromium carbide, quenching the austenitized cold-rolled steel sheet to room temperature, subzero treating the quenched resultant at a temperature of −70° C. for 10 seconds to 5 minutes to transform remaining austenite to martensite, and tempering the resultant at 400 to 600° C. for 30 minutes to 2 hours.
 13. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 3, wherein a batch annealing process to prepare the martensitic stainless steel comprises: a first soaking process to uniformly distribute Cu precipitates in a structure of the martensitic stainless steel; a second soaking process to uniformly distribute the chromium carbide in the structure of the martensitic stainless steel; and a third soaking process to spheroidize fine particles of the chromium carbide.
 14. The high-hardness martensitic stainless steel with excellent antibacterial properties according to claim 4, wherein a batch annealing process to prepare the martensitic stainless steel comprises: a first soaking process to uniformly distribute Cu precipitates in a structure of the martensitic stainless steel; a second soaking process to uniformly distribute the chromium carbide in the structure of the martensitic stainless steel; and a third soaking process to spheroidize fine particles of the chromium carbide. 